`maxreport`: bad types should return pretty errors...

[jmervine]

This is ugly! maxreport should qualify vaild types.

bash-3.2$ maxreport stats -t weekly --csv
Running weekly stats report.

panic: RWS\Exception: Resource not found at /jmervine/reports/stats.json/weekly

goroutine 16 [running]:
runtime.panic(0x3116e0, 0xc208201e90)
        /usr/local/go/src/pkg/runtime/panic.c:279 +0xf5
main.check(0x6211b0, 0xc208201e90)
        /Users/jmervine/.go/src/github.com/MaxCDN/maxcli/maxreport/maxreport.go:313 +0x4f
main.statsBreakdown(0xc20804e380)
        /Users/jmervine/.go/src/github.com/MaxCDN/maxcli/maxreport/maxreport.go:233 +0x219
main.stats(0xc20804e380)
        /Users/jmervine/.go/src/github.com/MaxCDN/maxcli/maxreport/maxreport.go:196 +0x43
main.main()
        /Users/jmervine/.go/src/github.com/MaxCDN/maxcli/maxreport/maxreport.go:188 +0x221

goroutine 19 [finalizer wait]:
runtime.park(0x19610, 0x569b10, 0x5685c9)
        /usr/local/go/src/pkg/runtime/proc.c:1369 +0x89
runtime.parkunlock(0x569b10, 0x5685c9)
        /usr/local/go/src/pkg/runtime/proc.c:1385 +0x3b
runfinq()
        /usr/local/go/src/pkg/runtime/mgc0.c:2644 +0xcf
runtime.goexit()
        /usr/local/go/src/pkg/runtime/proc.c:1445

goroutine 24 [runnable]:
net/http.(*persistConn).readLoop(0xc20804c160)
        /usr/local/go/src/pkg/net/http/transport.go:868 +0x829
created by net/http.(*Transport).dialConn
        /usr/local/go/src/pkg/net/http/transport.go:600 +0x93f

goroutine 17 [syscall]:
runtime.goexit()
        /usr/local/go/src/pkg/runtime/proc.c:1445

goroutine 25 [select]:
net/http.(*persistConn).writeLoop(0xc20804c160)
        /usr/local/go/src/pkg/net/http/transport.go:885 +0x38f
created by net/http.(*Transport).dialConn
        /usr/local/go/src/pkg/net/http/transport.go:601 +0x957